Should I be able to balance a nickel on the V12?

I just bought an ‘87 XJS. My first Jag, the same year, model and colour combination my dad had 30 years ago. Lifelong ambition achieved.

It’s not quite smooth enough to balance a coin on like in some YouTube videos. Should I care? Is there anything I can do?


Read @gregma posts that should give you a starting point, and maybe even a finishing point.

ha, i’m nowhere near balancing a nickel on my V12! I can balance a cup of coffee!


Funny you guys are talking about that at this time … I was just answering questions to someone who eyed Superblue in a store parking lot a few days ago, asking about what is the big advantage(s) (supposedly) of the V-12s vs. the 4.0s, given they both have about the same 0-60 time and top speed #. I told him the only advantage I know is that supposedly the V-12s are SO “smooth”, and that you can put a cup of a beverage on the hood w/o visible vibration to the contents. :laughing: Still, as I told him, I’d trade that characteristic for the better reliability of the 4.0s any day.

You are also supposed to be able to put the manual V12s in top gear and then start the engine and pull away.

IIRC my 66 Oldsmobile Toronado was so smooth we balanced a coin on the 425 ci engine while idling.

I balanced a loony ( Canadian dollar ) on my V-12 when I first got the car. Then I changed my spark plugs and ignition module this summer, haven’t been able to do it since. I have that annoying stumble every once in a while at idle. I might have a small exhaust leak at the front downpipes cat which might be the issue.

The only V12 I ever touched while running (85 HE) was so smooth I couldn’t tell it ran. It only moved when I revved it up… it got warm slowly and that was all. A coin might have been possible.

Similar here. Only time I’ve had engine idle that smooth is after changing plugs and cleaning throttle bodies.

It also idles that smooth after a good 45+ minute non-stop drive.

Obviously, things have to be just right - sensitive engine with antiquated fuel injection.

Heck, my school chum Roy’s Model A ford could do that.

when cops drove stick shift carsin the 40’sd, they hid and then used that technique to get the victim.

Work chum Bob, an ex cop used that when we raced from the park lot to the freeway. I finaly caught on as to why his VW got going faster than mine!!!

I think we’re talking different things, Carl. The trick of starting the car in high gear that Robin mentioned is anything but quick, you would never win any races doing that. It’s just interesting how smooth it is.

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You got that wrong. With the v12, you can pour a beverage into the engine bay and it will never find its way to the floor!


Didn’t someone (le mans?) rig the starter button to the door so he had a slight advantage?
First gear starter starts may help but top gear starts in a beetle would take a long time in a race :grin:

I’ve owned several Jags over the last 23 years, V12 and 6-cylinder. All of them have had idle quality problems. Sometimes an actual identifiable, correctable fault. More often just inconsistent. Sometimes smooth as glass, other times a bit of a tremble… varying from day to day or week to week.

I stopped beating my head against the wall a long time ago.



My XJR has a tremble/miss at idle, and I just live with it as well.

FYi, adjusting my throttle plates helped my idle a bit. They were a wee bit off - more gap at top than bottom. Loosening both screws and making sure top/bottom gap were the same actually helped.

I guess the air flow is critical? These V12s are sensitive!

LOLLLLLLLLLLLLLLLLLLLL, VK :grinning_face_with_smiling_eyes:

A good example of the need for the “Carl Principle” = so long as it does what it’s supposed to do, don’t fret over it … :grin: Glad you taught us that important lesson, Carl … :+1:

There’s good and bad days for all the engines I know. Some are more consistent and some less so. A v12 shouldn’t shake so the cup of water is the better test. A nickel on a good day is likely possible.


Thank you all for your replies.